Was Ten Hag's decision to dump De Gea and buy Onana the best decision? I don't think so. He has conceded 19 times in 11 matches. Pretty bad stats with fantastic transfer prices. Even Onana also did not only make one mistake resulting in a goal. I don't blame him all, because their defence is not very good either. But it seems that Ten Hag's football philosophy is not entirely running smoothly. Many supporters hope De Gea can return to Man United.
Ten Hag left De Gea leave the team because of some issues they had not because he is not good for the tea, which is a very wrong decision because De Gea is one of the best players in Manchester United last season because of his good saves that helped them qualify for the Champions League, and using Onana as his replacement is not a bad idea either because Onana was a good keeper at Ajax and Inter Milan as well, so I think if he is given some chances, he can improve his performance, but the errors he was making consistently that resulted in goals were the main reason Manchester United fans dislike him. But I think if he is given time, he can prove himself as a good replacement for De Gea.
Letting De Gea leave was a wrong call by Manchester United, even though they wanted to make Onana the 1st choice they would've retained him in the club just like Arsenal did with Ramsdale and Raya this season, De Gea would've helped them keep in some important matches like the UCL games against Bayern Munich and Galatasaray and United wouldn't had lost both especially the match against Galatasaray where Onana failed to catch weak shots and even allowed icardi to find his height it would be tough to do that against David De Gea.
Well i fail to understand why Onana is underperforming in Manchester United currently cause he was good under Ten Hag at Ajax and even did well last season at Inter, but is currently finding it difficult to adapt at Manchester United although he's good with his feet but that's not enough cause he'll need to start helping his team to save them from dropping points by catching simple shots.
But you are assuming that De Gea wouldn't have made any mistakes, but in reality he has made mistakes more often in recent times. Onana's start for Manchester United was far from perfect, but Ten Hag also played games with De Gea and I don't know they parted ways, but I don't know if De Gea was still interested in staying. When Ten Hag decides against a player, he almost has ideological reasoning and doesn't allow a player to quickly get back into the game. They finally need a world class coach again and for me Ten Hag is definitely not one of those.